In an electrophotographic modular printing machine of known type, such as for example the NexPress 2100 printer manufactured by NexPress Solutions, Inc., of Rochester, N.Y., color toner images are made sequentially in a plurality of color imaging modules arranged in tandem, and the toner images are successively electrostatically transferred to a receiver sheet adhered to a transport web moved through the modules. Commercial machines of this type typically employ intermediate transfer members in the respective modules for the transfer to the receiver member of individual color separation toner images.
In a modular machine of this type, sequential lay-down of color separation toner images onto the receiver sheet generally gives rise to a space charge within the stack of as yet unfused toner particles. When at least two previously transferred toner layers are already stacked one upon the other on the receiver member from transfer by prior modules, it is noted that certain defects can occur in the previously deposited toner layer farthest away from the surface of the receiver member. These defects can take the form of mottle covering the whole affected area or bands of mottle. The defects result from back-transfer of toner particles to an intermediate transfer member from this outermost previously deposited toner layer.
As an example, when a receiver member has magenta and cyan toners transferred thereon (in the third and fourth modules of the machine which includes successive modules or stations for black, yellow, magenta, cyan and clear toner) so as to make a final blue color in a large solid area of an image frame, the back-transfer defects can occur when the receiver member moves through the clear toner depositing module when the selected mode of operation is for a print with no clear toner covering the entire image. In a printer having a fifth toner depositing station for depositing clear toner, it is desirable to have the operator be free to select whether or not clear toner is desired as the final coat. The provision of a clear toner overcoat is desirable for providing protection of the print from fingerprints and reducing certain visual artifacts. However, a clear toner overcoat may add cost and may reduce color gamut of the print, so it is therefore desirable to provide for operator/user selection to determine whether or not a clear toner overcoat will be applied to the entire print.
In order to prevent back-transfer of the toner to the clear toner intermediate transfer roller, it may be possible in certain machines to provide for retraction of the intermediate transfer roller from engagement with the receiver member. However this option adds complexity and thus cost to a printer with this feature. Another approach for reducing back-transfer has been suggested by Rakov et al., in commonly assigned U.S. Patent Application Ser. No. 60/567,219 filed on Apr. 30, 2004, entitled “TONER TRANSFER TECHNIQUE” wherein transfer control current is combined with information derived using process control conditions to inhibit back transfer. Inherent in this solution is the depositing of toner where required for the particular color and applying a suitable transfer current. No indication is provided with regard to back-transfer to the intermediate transfer roller when the selected mode of operation is for non-covering of the overall image by the clear transfer toner.